The present invention relates to a process for the continuous production of ammonium thiosulphate.
It is known to produce aqueous solutions of ammonium thiosulphate (ATS) by reacting a solution of ammonium sulphites with sulphur in solid or liquid form, or with sulphides or polysulphides typically in aqueous solution, as described in Kirk-Othmer Encyclopedia of Chemical Technology, 4th edition, 1997, vol. 24, page 62, and in U.S. Pat. Nos. 2,412,607; 3,473,891; 3,524,724 and 4,478,807.
It is also known from U.S. Pat. No. 3,431,070 to produce ATS in a continuous process from gaseous feed streams comprising H.sub.2 S, NH.sub.3 and SO.sub.2. By the process of this patent, ATS and sulphur is produced from a first feed gas stream comprising H.sub.2 S and NH.sub.3 and a second feed gas stream comprising SO.sub.2 in a process comprising three absorption steps. In a first absorber, NH.sub.3 and H.sub.2 S in the first feed gas stream are separated in a H.sub.2 S off-gas stream and a NH.sub.3 -rich solution of ATS. The main part of the solution is passed to a second absorber, in which it is contacted with the SO.sub.2 -rich feed gas stream under formation of off-gas that is vented and a solution rich in ATS and ammonium sulphites, which in a third absorber is contacted with the H.sub.2 S off-gas from the first absorber and, optionally, with additional H.sub.2 S. After removal of sulphur being formed in the third absorber, the major part of the ATS-solution formed in the third absorber is recycled to the first absorber, while the minor part is mixed with a fraction of the NH.sub.3 -rich solution of ATS formed in the first absorber forming the product solution of ATS.
The major disadvantage of this process is that off-gas from the third absorber contains high concentrations of H.sub.2 S being emitted from the process.